1. Field of the Invention
The present invention relates to a method of operating a communications system in which secondary stations are able to roam in and out of respective radio coverage areas of a plurality of fixedly sited primary stations, the primary stations regularly transmitting idle beacon signals which are detectable by in range secondary stations operating in an idle locked state.
The invention also relates to such a communications system and to a secondary station for use therein.
2. Description of the Related Art
For convenience of description the invention will be described with reference to the DECT (Digital European Cordless Telecommunications) system, which comprises a plurality of time division duplex frequency channels which are accessed using a time division multiple access (TMDA) protocol. More particularly DECT comprises, for voice communication, one or more geographically separated primary or fixed base stations each having radio transceiving means defining a cell and a connection for example by landline, to the public switched telephone network (PSTN). One or more secondary or transportable, for example hand portable, stations having radio transceiving means are able to communicate by way of a radio link with an in-range primary station. The European Telecommunications Standards Institute (ETSI) currently provides that DECT shall have ten radio carrier channels, each divided into successive time frames of 10 milliseconds duration. Each frame is further divided into 24 equal time slots (or physical channels) which provide 12 time division duplex (TDD) channels termed duplex voice channels. The TDD arrangement is such that the nth and (n+12)th time slots, where n is an integer between 1 and 12, are a forward and reverse physical channels constituting the duplex voice channel. Each such pair of physical channels is capable of carrying one duplex digitized speech conversation or data at a rate of 1.152 Mbits/sec.
In setting up a voice call between a primary and a secondary station, a duplex voice channel is assigned to the transaction. The assignment of the duplex voice channel in any one of the radio carrier channels is by the method of dynamic channel allocation whereby a secondary station, taking into account its radio environment as determined by monitoring the average interference in each of the 120 pairs of physical channels, negotiates with the primary station for access to the best duplex voice channel currently available under the control of the primary station.
Currently the DECT specification requires base stations to continue transmissions on their last active forward physical channel so as to provide the system functions of broadcasting the basic system information, base station identity, and a frequency and frame timing reference. For convenience of description such transmissions will be called "normal idle beacon" transmissions. A secondary station in range of the primary station can remain tuned to the active forward physical channel but can power down. However the secondary station is powered-up for one frame (10 ms) in every 16 frames (or 160 ms) in order to be capable of receiving any paging transmissions addressed to it. Also, ideally the secondary station checks every 2 to 5 seconds whether it is correctly tuned to, and in synchronism with, the strongest or best primary station. A reason given for having these normal idle beacon transmissions is to save power at the secondary stations. However, when a secondary station wishes to make a call it must be able to scan rapidly the physical channels to obtain the best channel and then establish frame and slot synchronization using the normal idle beacon transmissions.
When a secondary station leaves its cell it may continuously scan all the duplex voice channels for another suitable primary station's idle beacon transmission. This continuous scanning by the secondary station consumes battery power which is wasted if the secondary station has moved out of the system's coverage area for a long period. One option to save battery power is to switch off the secondary, station but in order to do that the user must have realized that he is outside the coverage area. Also the user has to remember to switch on the secondary station when coming into range of a primary station to enable it to scan all the channels.